I know that you probably shouldn't set an attribute (or anything else) to undefined, but this is handy when copying attributes from another element:

$('<div>')
.attr('id', otherdiv.attr('id')) // if otherdiv doesn't have an id, we blow up here.
.attr('class', otherdiv.attr('class'));

This worked correctly before jQuery 1.6 but broke when attr('doesnotexist') began returning undefined.

I would have sent a patch, but attr() then immediately defers to access() which has no way to find out whether attr() was called with one or two parameters, so fixing this would be a huge refactoring effort I definitely don't feel comfortable with.